Refractory mold material for cast metal products



Patented Mar. 4, 1941 4 PATENT OFFICE REFRACTORY MOLD MATERIAL FOR CASTLIETAL PRODUCTS Cornell Joel Grossman, Millburn, N. J

5 Claims.

This invention relates to refractory materials for cast metal productsand more particularly to refractory mixtures adapted for the forming ofmolds for casting molten metal therein and more specifically torefractory mold material adapted for use in the manufacture of castmetallic dentures.

One of.the objects of the present invention is to provide a refractorymixture adapted to be cast while in the plastic state about a form orpattern defining the desired cast metal product and which in the as castand as fired condition is dense and strong especially at the surfacedefined by the pattern and substantially free from surfaceirregularities and of loose. material r more fully hereinafterdisclosed.

In accordance with these objects, I have discovered that a mixture offine and coarse silica, preferably one consisting of particles of a sizepassing about 20 mesh with from 20-40% thereof passing about 200 mesh isthe most satisfactory refractory for the manufacture of molds for castmetallic dentures. The difficulty in the use of this refractory is inthe bonding of this material at low temperatures and at hightemperatures, or in the as cast and pre-fired" condition respectively.

In the forming of molds for east metallic dentures, a pattern is firstformed of wax or similar material of the size, shape and configurationof the metal denture desired. The mold material is then formed into aplastic mass and cast about the pattern preferably by vibrating the samethere around and is backed up with suitable refractory backing materialsuch as plaster of Paris. Theimolded refractory then is dried and isslowly heated to a temperature approximately 1500 F. to thoroughlyconsolidate the same and then is allowed to cool slowly to a lowertemperature known as the pre-heat temperature desired for casting themolten metal in the mold. The molten metal then is cast, preferably bycentrifugal casting means, into the pre-fired and preheated mold.

In the forming of cast metallic dentures it is highly desirable that themold material employed in addition to surface density and smoothnessshall evidence substantially no shrinkage or expansion within thetemperature range atmospheric to about 1500'- F. and also that thebinding agent employed for strength in the as cast" No Drawing.Application July 17, 1940, Serial N0.-345,981

and as fired condition shall be substantially free from carbon andsulfur. I

'I have discovered that by mixing magnesium trisilicate with the silicaand by employing an acid phosphate solution in the forming of theplastic solution in the forming of the plastic refractory mass to moldaround the pattern, I may obtain a relatively quick setting bond for thesilica in the as cast condition of adequate strength for the purposewhich on firing retains its strength up to the maximum firingtemperatures usually employed, namely about 1500 F. The resultant firedrefractory product meets all the exacting requirements for molds foreast metallic dentures, particularly the non-expanding and non-shrinkingrequirement.

The exact reaction involved in the use of these materials is not clearlyapparent. The magnesium trl-silicate appears to react initially while inthe plastic or moistened condition with the acid phosphate to formcomplex magnesium-silico-phosphates which serve as a low temperaturebonding agent. It is possible that the phosphoric acid reacts also, atleast superficially, with the silica refractory to form silicophosphates. On firing the silico-phosphates appear to retain theoriginal bond even at temperatures materially above 1500 F.

The silica refractory employed is preferably prefired or pre-shrunksilica of approximately the maximum density obtainable, for exampleelectrically fused silica. Natural flint silica, however, has provensatisfactory for the present invention.

The magnesium tri-silicate is preferably freshly precipitated'andunfired silicate of the maximum chemical reactivity towards phosphoricacid, although various admixtures of pre-fired and freshly precipitatedsilicate may be employed without departure from the present invention,as one skilled in the art may perceive. In place of magnesiumtri-silicate or in partial substitution therefor other silicatecompounds reactive with phosphoric acid to form refractorysilico-phosphate compounds, may be employed without departure from thepresent invention, as one skilled in the art also will perceive.

The acid phosphate solution employed may be .varied widely with respectto composition and the desired strength and density in refractory in the"as cast condition. The chemical reactivity of these acid phosphatecompounds may be increased, if desired, by additions of an acid to thewater solution of the same, preferably phosphoric or hydrochloric acid.To obtain the desired high temperature bond, the metal component of theacid phosphate preferably is one pre-fired condition by additions ofmagnesium oxide to the refractory mixture of silica and magnesiumtri-silicate. Preferably such oxide should be freshly precipitated oxideto obtain its maximum chemical reactivity. The oxide appears to reactwith the phosphoric acid or acid phosphate compounds to form acidmagnesium phosphates which evidence excellent bonding properties atleast superficially on the silica refractory augmenting that obtained bythe siilco-phosphate compounds formed by the reaction of the trisilicateand acid phosphate.

Additions of zirconium or of titanium double silicates such as thezirconium magnesium double silicate appear to materially increase thepre-fired strength of the mold material and to give added surfacedensity and coherence to the same.

From the above disclosure it is believed apparent that the presentinvention may be wideto 25% phosphoric acid, to obtain a fast settingmold material which on firing will evidence excellent strength anddensity'for most purposes. As an illustration, a mixture consisting of90% electrically fusedsilica with about 80% of the grains passing 20mesh but retained on 200 mesh and about 20% of the grains passing 200mesh, with about 10% magnesium tri-silicate as a fine powder (each dryweight) when intimately mixed together in the dry state and then formedinto a plastic mass with a 10% phosphoric acid solution will set to arelatively hard and dense mass within a few minutes. The mold when firedat 1500" F. for several hours will be found to produce accuratelydimensioned castings of the pattern employed with any of the metalscommonly employed in the forming of metallic dentures.

By lowering the tri-silicate content of this mixture to about 5% thesetting time of the mass will be lengthened to about hour; by increasingthe phosphoric acid strength to 20% the mold strength will be materiallyincreased and the setting time decreased; by substituting an acidphosphate, such as Al or Mg acid phosphate in part or in whole for thephosphoric acid disclosed, I may obtain improved mold strength anddensity in the as cast condition without materialshortening of thesetting time, or material alteration of the strength and density of thesame in the as firedcondition.

Alternatively, by incorporating magnesium oxide or similar refractorymetal oxides reactive with phosphoric acid and phosphates into therefractory mixture in addition to the silica and tri-silicate compoundsspecified above, in amounts ranging from small but effective amounts" upto about 5%, the normal mold strength andij; ensity, particular surfacedensity in the as fired condition may be beneficiated. Acid solublerefractory metal hydroxides are substantial equivalents for the oxide.

As a further improvement, particularly with respect to the pre-firedstrength and density of the mold, I may add to the refractory mixtureabove given from small but effective amounts up to about 30%.-,(dryweight) of zirconium or titanium double silicate'compounds, such as thezirconium magnesium silicate compound, ground to pass about 200 mesh,preferably replacing equivalent amounts of the' fine silica contentspecified. Such additions materially increase the pre-fired strength anddensity of the mold and are particularly desirable to improve thesurface strength and density.

Wherein in the above specific embodiment I have specified a mixtureconsisting of coarse silica and 20% fine silica, it is not to beconstrued'that I am limited thereby as anymixture of coarse and finesilica wherein the fine silica passing 200 mesh is within the range20-40% and the coarse silica size is below about 20 mesh is within thescope of the present invention.

Having hereinabove described the present in vention generically andspecifically and given the'specific embodiment thereof withmodifications thereof, it is believed apparent that the presentinvention may be widely varied without essential departure therefrom andall such modifications and departures are contemplated as may fallwithin the scope of the following claims:

What I claim is:

1. A refractory mixture for use with phosphoric acid or a similar binderto form molds for casting molten metal therein, said mixture compris--ing at least 60% of a mixture of fine and coarse silicaand magnesiumtri-silicate, the latter constituent being presentin amount ranging fromsmall buteifective amounts up to about 20%.

' 2. A refractory mixture for use with phosphoric acid or a similarbinder to form a mold for cast metallic dentures, said mixturecomprising of about 80% silica passing about 20 mesh and 5 retained onabout 200 mesh, about 20% silica.

passing about 200 mesh and about 10% mag- I nesium tri-silicate.

3. A refractory mixture for use with phosphoric acid 'or asimilar binderto form a mold for easting molten metal therein, said mixture comprisingat least 60% of a mixture of fine and coarse silica and magnesiumtri-silicate.

4. The method of forming a refractory mold for cast metallic dentureswhich comprises forming a mixture comprising at least 60% of silica I,

and magnesium tri-silicate, forming the mixture into a plastic mass withphosphoric acid or similar binder and flowing the plastic mass over. thesurface of a pattern in a pluralityof coat-' ings to the depth andthickness desired, allowing the coatings to set, drying the coatings andthen

